1. Field of the Invention
The present invention is concerned with the problems of interaction occurring in power transmission systems which incorporate turbine generators.
By subsynchronous frequencies is meant frequencies which are below the system frequency, which may be, e.g. 50 Hz or 60 Hz, and which may be produced in power transmission networks when the networks are subjected to disturbances of varying kinds. These subsynchronous frequencies are very troublesome when turbine generators are connected to the network. This is associated with the fact that a turbine generator is comprises of a generator and several fast rotating turbines (e.g. high pressure turbines--first and second low pressure turbines) which are provided with shafts which together with the large rotating masses in turbines, generators and magnetizing rotors, are able to form oscillators on the basis of torsion oscillations in the shafts, with an often complicated array of resonance frequencies within the subsynchronous range. These mechanical resonance circuis are connected electrically direct to the network, through the generator.
This problem has become highly noticeable, particularly as a result of the series compensation with capacitors in long transmission lines and also particularly as a result of an increase in the degree of compensation, in that the amount of energy stored in the capacitor batteries is so large, this energy being available for the generation of subsynchronous resonances in the network in the event of a disturbance. Such a disturbance oscillation is coupled, through the generator, directly to the mechanical system of the turbine generator, which when the resonance frequencies are adapted to the resonance frequency of the network, will be caused to oscillate at such an amplitude as to result in possible damage to the shafts of said system.
2. Related Art
This problem has been the subject of various solutions, where attempts have been made to dampen the resonance oscillations by incorporating resistances in the power transmission lines.
In the case of one such damping system described and illustrated in Swedish Published Specification No. 8000104-3 (446.289) an ohmic resistance which is coupled in parallel with a series resonance circuit tuned to the system frequency is connected in series with each phase in the block transformer and/or generator of a turbogroup. These subsynchronous oscillation damping circuits may either be activated constantly or may be activated in response to the control of a disturbance sensor.
In the case of another system according to U.S.-A-4,434,376 or 4,355,241 a damping resistance in the network circuit is activated when a sensor detects a time distance between zero crossings which exceeds those normal for the network frequency, therewith indicating the presence of a subsynchronous resonance.
This known system is therefore often very expensive, especially when the system function requires the presence of sensors and switching means.